The field of the present invention pertains to semiconductor fabrication processing. More particularly, the present invention relates to a method and device for more effectively mounting a backing film onto a polisher head in a chemical mechanical polishing machine.
Much of the power and usefulness of today""s digital integrated circuit (IC) devices can be attributed to the increasing levels of integration. More and more components, such as resistors, diodes, transistors and the like, are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal and takes the shape of a solid cylinder. This crystal is then sawed, similar to slicing a loaf of bread, to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components are commonly defined photographically through a process known as photolithography. Very fine surface geometries can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built up layers, with each layer having its own components, differing interconnections, and stacked on top of the previous layer. The resulting topography of these complex ICs often resemble familiar terrestrial xe2x80x9cmountain ranges,xe2x80x9d with many xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern defining the various components is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using the optical means of the photolithography tool, and is imprinted into the photosensitive layer. To build ever smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer or, stated differently, optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, defined by the xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d as described above, exaggerate the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (e.g., planarized) surface will allow for extremely small depths of focus and, in turn, will allow the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) has been widely used in semiconductor fabrication and processing to planarize a silicon wafer and/or to removal blanket material thereon. Typically, CMP involves removing a sacrificial layer of dielectric material using mechanical contact between the wafer and a moving polishing pad that is saturated with a chemical known as polishing slurry. Polishing minimizes the height differences between high and low spots on the wafer, since higher spots (xe2x80x9chillsxe2x80x9d) are removed faster than lower spots (xe2x80x9cvalleysxe2x80x9d) during the polishing process. Polishing in this manner is the only technique that produces a smooth topography on a processed wafer when it is examined on a millimeter scale. That is, the wafer is essentially flat (e.g., planarized) when measured over the distance of a millimeter, and angles between high and low spots remaining after CMP are generally much less than one degree.
Regardless of its exact design, a typical CMP machine has a polisher head and carrier mechanism that holds the wafer during the polishing process. Inside the polisher head, there is usually a backing film between the wafer and the backing plate of the carrier. On the backing plate, there is a hole pattern that comprises numerous air holes and allows the user to adjust the magnitude and direction of back pressure that is applied to the wafer. The backing film has a hole pattern corresponding to that on the backing plate of the carrier. The perforated backing film is attached to the backing plate so that their respective hole patterns align. As such, proper pressure as specified by the user can be applied by the polisher head to the wafer through both the backing plate and the backing film.
Traditionally, in order to prevent the backing film from rotating relative to the carrier during polishing operations, which would result in misalignment of the perforations on the backing film with the air holes of the backing plate, an adhesive is applied to the contact surface(s) of the backing film and/or the backing plate to hold the backing film in place. However, this method is problematic because over time the adhesive on the backing film/backing plate interface wears out. As a result, the backing film begins to slip in the opposite direction of the rotating carrier. The tendency of slipping is especially strong during those processes that require much mechanical action. Eventually, the perforations of the backing film are no longer aligned with the air holes of the backing plate, thereby partially or even completely blocking off the air flow that is applied by the polisher head to the wafer. Consequently, the desired pressure cannot be correctly applied to the wafer.
Another problem with the use of adhesive relates to certain high temperature polishing processes. During such processes, in which the operating temperature can well exceed 100xc2x0 F., the performance of the backing film could be degraded due to the presence of the adhesive. Moreover, a strong adhesive is usually used to affix the backing film in order to prevent slipping. Yet, the use of strong adhesives creates yet another problem during carrier rebuilds, as the adhesives can be very difficult to clean off. In addition, the use of thicker adhesives could possibly clog the holes on the backing film and/or the backing plate, thus again negatively affecting the performance of the carrier to apply the correct back pressure on the back side of the wafer.
In sum, these shortcomings of the prior art solutions adversely affect the performance of the CMP machine. More particularly, they degrade the expected lifetime of the backing film and/or the carrier. At the least, once the slipping as described above has started, the user would have to readjust or replace the misaligned backing film in order to properly control the pressure that is applied to the wafer being processed in the CMP machine.
Thus, what is needed is a mechanism which reliably prevents the backing film in the polisher head of a CMP machine from slipping during polishing operations. Further, what is needed is a mechanism which achieves the above without the drawbacks described above, namely, the short useful life expectancy of the backing film and/or the carrier, the difficulty of performing carrier rebuilds, the clogging of air holes and/or backing film perforations, and the performance degradation due to high temperatures. The present invention provides a solution that meets the recited needs.
It would be advantageous to provide a mechanism which reliably prevents the backing film in the polisher head of a CMP machine from slipping during polishing operations. More particularly, it would be desirable to provide a mechanism which achieves the above without imposing a short useful life expectancy of the backing film and/or the carrier, or rendering carrier rebuilds difficult to perform. It would also be beneficial to provide a solution that does not cause air holes and/or backing film perforations to be clogged, or suffer from performance degradation during high temperature processes.
Accordingly, the present invention provides a non-slip polisher head backing film for use in a CMP machine. More particularly, the present invention reliably prevents slipping of the backing film from occurring for a prolonged period, so that the backing film and the carrier have an extended useful life. The present invention is also immune from performance degradation during high temperature processes. Moreover, the present invention has no adverse effect on carrier rebuilds and does not cause clogging of air holes or backing film perforations.
More specifically, in one embodiment, the present invention polisher head backing film/backing plate assembly includes a backing film with a hole pattern as well as holding pins that protrudes from one of the film""s surfaces. The backing film/backing plate assembly also includes a backing plate that has a corresponding hole pattern and receiving holes such that the hole patterns on the backing film and backing plate can be aligned. The receiving holes can receive the holding pins of the backing film and are positioned such that when the hole patterns are aligned, the holding pins and the receiving holes are aligned as well. Thus, upon proper alignment, the holding pins can be inserted into the receiving holes and the backing film cannot move relative to the backing plate once the holding pins are properly inserted. This embodiment of the present invention thus not only provides for the application of proper pressure by the polisher head to the wafer being polished, but also eliminates the problems associated with the use of adhesives as a means of backing film attachment as described above.
In one embodiment, the holding pins are located along the edge of the backing film, where they can best resist the tendency of the backing film to rotate or slip relative to the backing plate during polishing operations. In a currently preferred embodiment, the holding pins protrudes substantially vertically from the surface of the backing film.
In yet another embodiment, the present invention also provides a locking mechanism which can be engaged after the holding pins are inserted into the receiving holes in order to further ensure that the holding pins are locked in place and will not accidentally dislodge from the receiving holes.